To eliminate an S/H circuit by arranging a line sensor while inclining it from a perpendicular line for the scanning axis of an optical scanning means and delaying measurement time and then obtaining an infrared radiation energy from each infrared detection element.

An infrared element arrangement axis 16 of a line sensor 4 is inclined from a perpendicular orthogonal axis 21 by a specific angle value for a scanning axis 15 in an X-axis direction, and a line sensor 4 is arranged since each element measures a line 17 to be measured successively in the direction of the element array axis 16. After an element 4a measures an infrared energy, an inclination angle is set to an angle that an element 4b measures after a measurement time difference (t) when data processing is completed. Then, electrical signals STa-STn with the delay time (t) are taken out of an inclined line sensor 4' and is supplied to the linearizer of a temperature signal processing means after being delayed by data processing time (t), thus eliminating a plurality of sample/hold (S/H) circuits corresponding to each element between the linearizer and a multiplexer.